While a lithium ion battery has a feature that the current output property is good because it has low impedance, a large current (100 A, for example) flows when a short circuit occurs. For this reason, a sufficient measure against a case where a short circuit occurs to a lithium ion battery is needed. In safety testing about transportation of a battery, a protection test to a short circuit has become indispensable, and measures to a short circuit are very important.
Up to now, a structure in which a switch including an FET (Field effect transistor) and the like is provided has been proposed for the purpose of battery protection and the like against a short circuit. In this structure, when electric current of a value equal to or larger than a value set in advance flows, battery protection is carried out by the switch operating and cutting off the electric current.
However, short-circuit current may exceed 1000 A before the switch cuts off the short-circuit current. Accordingly, the switch needs to have a current limiting capability (permissible current) no smaller than 1000 A. However, a switch of a large permissible current is very expensive and is a factor to largely increase the cost of a device.
About this issue, there is proposed in Patent Literature 1 (PTL1) a technology in which an overcurrent detecting means equipped with a delay circuit is provided, and ON time of a discharge control switch is controlled by the overcurrent detecting means. At that time, the delay circuit has a function to change a delay time continuously (in an analog manner). Then, it is made such that, in response to a sudden change of a state (increase of electric current) in the discharge control switch, the discharge control switch can become OFF state quickly. As a result, a large current is prevented from flowing for a long time in the discharge control switch, and destruction of the discharge control switch is prevented.
Patent Literature 2 (PTL2) relates to charge control of a secondary battery. Proposed is that a voltage between the terminals of a cell of a secondary battery is read and then input to an analog-to-digital converter, and, from an input value from the analog-to-digital converter, a charge control unit performs a protection operation, such as blocking off of an FET interposed in the charge path and so on, against abnormality such as a short circuit between the terminals and the like.
Patent Literature 3 (PTL3) relates to a detecting device of a short-circuit flaw in an electroplating line for a steel plate. Since, when a short-circuit flaw occurs to a steel plate in electroplating, flowing electric current changes rapidly, it is proposed to detect this electric current by a current detection means such as a shunt and the like, differentiate output of the current detection means, and determine by a comparison means whether the differentiated value is within a predetermined scope.